The present invention relates to annular illuminators and, more particularly, to a 45.degree. annular illuminator.
When examining a surface critically to judge its color or to discern an image or pattern, it is preferable to orient the illuminant so the surface is well illuminated but no light is specularly reflected to the eye. If a plane surface is viewed normally and illuminated at 45.degree. to the normal, these conditions can be met. Indeed, several standardizing bodies have chosen 45.degree. illumination for the measurement of certain reflecting characteristics of surfaces. See, e.g., Colorimetry, Official Recommendations of the International Commission on Illumination, Publication CIE No. 15 (E-1.3.1) 1971, Paragraph 1.4; 45-Deg. 0-Deg Directional Reflectance of Opaque Specimens by Filter Photometry, Test for ASTM E97; American National Standard Diffuse Reflection Density, ANS PH2.17-1958.
Various types of illuminating systems are known in the prior art. U.S. Pat. No. 4,022,534 to Kishner has a 45.degree./0.degree. illuminator/collector geometry and employs a wedge-shaped diffuser and a cylindrical reflector to obtain 45.degree. illumination. U.S. Pat. No. 1,445,306 to Epstein shows a reflector having a light source interposed between a semi-ellipsoidal reflecting surface and a spherical surface. U.S. Pat. No. 3,982,824 to Rambauske discloses a catoptric lens arrangement utilizing a primary mirror formed by rotating a portion of a parabola within a secondary mirror formed by a portion of an ellipse. U.S. Pat. No. 1,711,478 to Halvorsen, Jr. discloses a reflector having a parabolic region and two spherical regions of different radii. U.S. Pat. No. 3,257,574 to McLintic shows a reflector comprising a concave ellipsoidal section and a truncated concave spherical section. U.S. Pat. No. 3,893,754 to McInally discloses a mirror system utilizing paraboloid and ellipsoid mirrors in combination. U.S. Pat. No. 3,801,773 to Matsumi shows a reflector comprising two pairs of congruent, coaxial prolate spherical surface portions, each of which is disposed outwardly of the other. U.S. Pat. No. 3,449,561 to Basil et al. shows a mirror formed by revolving about a generating axis curved line segments which constitute in part portions of ellipses whose major axes lie at different acute angles to the generating axis. Finally, U.S. Pat. No. 4,002,499 to Winston discloses an energy collector comprising a pair of involute sections forming what is sometimes called a "gull-wing" solar collector.
In accordance with the present invention there is provided a highly efficient annular reflector comprising in combination a spheric mirror, an elliptic mirror and a circular cylindric mirror. Flux received directly from the source is reflected by an elliptic surface of revolution and then by the cylindric mirror. Flux not directed initially toward the elliptic mirror is first reflected by the spheric mirror and then by the elliptic and cylindric mirrors. This system is highly efficient since it utilizes all of the flux except that directed in a small angle about the axis of revolution. The annular illuminator of the present invention is useful for illuminating at angles of about 45.degree. as well as at angles of other than about 45.degree. such as angles between about 40.degree. and about 50.degree..